This invention relates to a thermal recording system. More specifically, this invention relates to a thermal recording system wherein a thermal imaging medium is heated imagewise prior to being brought into contact with a receiver material.
Printers based upon a process known as xe2x80x9cthermal wax transferxe2x80x9d, or, more correctly, xe2x80x9cthermal mass transferxe2x80x9d are available commercially. Such printers use an imaging medium (usually called a xe2x80x9cdonor sheetxe2x80x9d or xe2x80x9cdonor webxe2x80x9d) which, in the case of a color printer, comprises a series of panels of differing colors. Each panel comprises a substrate, typically a plastic film, carrying a layer of fusible material, conventionally a wax, containing a dye or pigment of the relevant color. To effect printing, a panel is contacted with a receiving sheet, which can be paper or a similar material, and passed across a thermal printing head, which effects imagewise heating of the panel. At each pixel where heat is applied by the thermal head, the layer of fusible material containing the dye or pigment transfers from the substrate to the receiving sheet, thereby forming an image on the receiving sheet. To form a full color image, the printing operation is repeated with panels of differing colors so that three or four images of different colors are superposed on a single receiving sheet.
Thermal wax transfer printing is relatively inexpensive and yields images which are good enough for many purposes. However, the resolution of the images which can be produced in practice is restricted since the separation between adjacent pixels is at least equal to the spacing between adjacent heating elements in the thermal head, and this spacing is subject to mechanical and electrical constraints. Also, the process is essentially binary; any specific pixel on one donor panel either transfers or does not, so that producing continuous tone images requires the use of dithering, stochastic screening or similar techniques to simulate continuous tone. One version of thermal wax transfer, called variable dot wax transfer, creates gray scale at the pixel level by creating a variable dot. This is accomplished by using a variable dot printhead, which has smaller heating elements, which creates a more peaked thermal gradient in the media. The longer heat is applied at the pixel the larger is the dot formed. It is not necessary to use halftoning with this technique. However, one problem with this technique is that it becomes very difficult to transfer small dots which results in grain and in the loss of detail in the low density regions.
Finally, some difficulties arise in accurately controlling the color of the images produced. The size of the wax particle transferred tends to vary depending upon whether an isolated pixel, or a series of adjacent pixels are being transferred, and this introduces granularity into the image and may lead to difficulty in accurate control of gray scale. Also, the size of the wax particle transferred depends on the thermal properties and surface roughness of the receiving material. Local nonuniformities in these properties in the receiving material introduce granularity into the image. This effect, in turn, requires expensive specialized receiving materials for high quality images. In addition, any given pixel in the final image may have 0, 1, 2, 3 or 4 superimposed wax particles, and the effects of the upper particles upon the color of the lower particles may lead to problems in accurate control of color balance.
Printers are also known using a process known as xe2x80x9cdye diffusion thermal transferxe2x80x9d or xe2x80x9cdye sublimation transferxe2x80x9d. This process is generally similar to thermal wax transfer in that a series of panels of different colors are placed in succession in contact with a receiving sheet, and heat is imagewise applied to the panels by means of a thermal head to transfer dye from the panels to the receiving sheet. In dye diffusion thermal transfer processes, however, there is no mass transfer of a binder containing a dye; instead a highly diffusible dye is used, and this dye alone transfers from the panel to the receiving sheet without any accompanying binder. Dye diffusion thermal transfer processes have the advantages of being inherently continuous tone (the amount of dye transferred at any specific pixel can be varied over a wide range by controlling the heat input to that pixel of the panel) and can produce images of photographic quality. However, the process is expensive because special dyes having high diffusivity, and a special receiving sheet, are required. Also, this special receiving sheet usually has a glossy surface similar to that of a photographic print paper, and the glossy receiving sheet limits the types of images which can be produced; one cannot, for example, produce a image with a matte finish similar to that produced by printing on plain paper, and images with such a matte finish may be desirable in certain applications. Finally, problems may be encountered with images produced by dye diffusion thermal transfer because the highly diffusible dyes tend to xe2x80x9cbleedxe2x80x9d within the image, for example, when contacted by oils from the fingers of users handling the images.
Finally, there is a thermal imaging system, described in, inter alia, U.S. Pat. Nos. 4,771,032; 5,409,880; 5,410,335; 5,486,856; and 5,537,140, and sold by Fuji Photo Film Co., Ltd. under the Registered Trademark xe2x80x9cAUTOCHROMExe2x80x9d which does not depend upon transfer of a dye, with or without a binder or carrier, from a donor to a receiving sheet. This process uses a recording sheet having three separate superposed color-forming layers, each of which develops a different color upon heating. The top color-forming layer develops color at a lower temperature than the middle color-forming layer, which in turn develops color at a lower temperature than the bottom color-forming layer. Also, at least the top and middle color-forming layers can be deactivated by actinic radiation of a specific wavelength (the wavelength for each color-forming layer being different, but both typically being in the near ultra-violet) so that after deactivation the color-forming layer will not generate color upon heating.
This recording sheet is imaged by first imagewise heating the sheet so that color is developed in the top color-forming layer, the heating being controlled so that no color is developed in either of the other two color-forming layers. The sheet is next passed beneath a radiation source of a wavelength which deactivates the top color-forming layer, but does not deactivate the middle color-forming layer. The sheet is then again imagewise heated by the thermal head, but with the head producing more heat than in the first pass, so that color is developed in the middle color-forming layer, and the sheet is passed beneath a radiation source of a wavelength which deactivates the middle color-forming layer. Finally, the sheet is again imagewise heated by the thermal head, but with the head producing more heat than in the second pass, so that color is developed in the bottom color-forming layer.
In such a process, it is difficult to avoid crosstalk between the three color-forming layers since, for example, if it is desired to image an area of the top color-forming layer to maximum optical density, it is difficult to avoid some color formation in the middle color-forming layer. Insulating layers may be provided between the color-forming layers to reduce such crosstalk, but the provision of such insulating layers adds to the cost of the medium. Print energy tends to be high, since the third pass over the thermal head to form color in the bottom color-forming layer requires heating of this layer through two superposed color-forming layers, and two insulating layers, if these are present. Finally, the need for at least two radiation sources to produce two well-separated wavelengths adds to the cost and complexity of the apparatus required.
Generally speaking, the prior art thermal imaging methods involve the application of heat by a thermal imaging head to the donor element while the donor element is in contact with the receiver material. This arrangement is not always completely satisfactory because the amount of energy needed to reach the required imaging temperature is affected by the receiver material, typically paper, and therefore the energy necessary is typically higher. Also, the image quality of the image formed may be adversely affected by non-uniform receiver layer surfaces.
U.S. Pat. No. 4,504,837 describes a method and apparatus for recording color images as color transfer superimposed laminations. In one embodiment described therein imaging is effected by applying heat to a transfer sheet while it is in contact with an ink ribbon to form an image on the transfer sheet and a base sheet is then laminated over the image on the surface of the transfer sheet. In another embodiment (see, for example, FIG. 9 and the discussion beginning at column 7, line 32) three separate color donor elements are imaged by three separate thermal heads before the donor elements are brought into contact with the receiver material but with a roller in contact with the back side of the donor element. Subsequently, the entire coloring layer of the donor element is stripped from the support layer and transferred to a base sheet. In the multicolor embodiment illustrated in FIG. 9 of the ""837 patent subsequent color layers are superimposed over the first transferred color layer.
As the state of the art advances and efforts are made to provide new thermal recording systems which can meet new performance requirements and to reduce or eliminate some of the undesirable characteristics of the known systems it would be advantageous to have a thermal recording system wherein the effects of the receiver material upon the energy requirements of the system and on the image quality of the images obtained can be significantly reduced or substantially eliminated.
It is therefore the object of this invention to provide a novel thermal recording system.
It is another object to provide a thermal recording system wherein the energy required for imaging is not affected by the receiver material on which the image is recorded.
It is still another object of the invention to provide a thermal recording system wherein a thermal imaging medium is heated by a thermal printing head prior to being brought into contact with a receiver material.
Yet another object of the invention is to provide a thermal recording system wherein one surface of a thermal imaging medium is heated by contact with a thermal printing head while the opposite surface of the imaging medium is in contact with air.
A further object of the invention is to provide a thermal recording system wherein an image formed in a thermal image-forming layer can be transferred to a receiver material without the application of any substantial additional heat.
Still another object is to provide a thermal recording system wherein an image formed in a thermal image-forming layer can be transferred to a receiver material and laminated over a previously transferred image or images formed in a thermal image-forming layer or layers without causing any appreciable undesirable further thermal development of the previously transferred image(s).
A further object is to provide a thermal recording system capable of high image quality which permits the use of a broad range of receiving materials.
These and other objects and advantages are accomplished in accordance with the invention by providing a novel thermal recording system wherein a specific area of a thermal imaging medium is imaged by being brought into contact with a thermal printing, or imaging, head and heated imagewise before that imaged area of the imaging medium is brought into contact with a receiver material. According to the invention, during the imagewise heat application step the area of the surface of the thermal imaging medium opposite to the area of the surface in contact with the thermal printing head is in contact only with air. The imaging medium is held firmly against the thermal printing head by tension. Subsequently, at least the imaged areas of the thermal imaging medium are transferred to a receiver material. Thus, the effect of the receiver material upon the energy required to attain the requisite imaging temperature is substantially or completely avoided.
According to a preferred embodiment of the invention, the distance between the point where any specific area of the thermal imaging medium is imaged by the thermal printing head and the point where that specific area of the imaged thermal imaging medium is transferred to a receiver material is selected such that no, or only very little, additional energy is needed to reach the required temperature for transferring the image formed in the thermal imaging medium to a receiver material.
In preferred embodiments of the invention, as will be described in detail below herein, the distance between imaging of the thermal imaging medium and transfer of the image formed in the imaging medium to a receiver material is a function of the length of the surface of the thermal heating element in the thermal printing head, i.e., the length in the print direction (the travel direction of the thermal imaging medium and the receiver element), and is measured from the center of the surface of the thermal heating element. The distance between application of heat to any point on the thermal imaging medium (referred to herein as the xe2x80x9cimage generation pointxe2x80x9d) to transfer of the image formed in that location to a receiver material (referred to herein as the xe2x80x9cimage transfer pointxe2x80x9d) is from about two to about six times the length of the surface of the thermal heating element.
The thermal recording system of the invention permits the use of less energy to attain the imaging temperature required by the thermal image-forming material in any particular instance. In a preferred embodiment, an imaging medium comprising a substrate carrying as the thermal image-forming layer a color-change layer which develops color upon heating is utilized and cross-talk between successively transferred, differently colored image-forming layers to form a multicolor image can be avoided without the necessity of fixing the previous image before transferring a subsequent image over it. Further, according to another preferred embodiment the imaged thermal image-forming layer can be transferred to a receiving element with only relatively little or no additional energy being required.